Cafe and restaurant owners aren't keen to let customers bring their own food, but many say one owner's decision to charge $40 for the privilege is over the top.

Southland District mayor Frana Cardno was caught out when she ate a banana at a Milton cafe where the owner had placed signs alerting customers to the $40 fee.

The mayor reportedly ate her own banana because she suffered from lactose intolerance, sparking debate on the issue of accommodating food allergy sufferers in restaurants.

A NZ Herald poll found 20 North Island cafes and restaurants adhered to the same rule but most thought trying to charge customers to eat their own food was unreasonable.

Instead they made allowances for babies and toddlers, and people with food allergies, and asked all other customers to put the homemade or bought elsewhere food away while in their eatery.

Lawrenson Group chief executive John Lawrenson, who owns six restaurants in Hamilton, said it was rude of customers to try to sneak their own food into his hospitality businesses.

"We're paying rent and staff. It's like letting people live in your house for free," Mr Lawrenson said.

"We see it a fair bit, people just walk into our pubs [which encompass restaurants] and sit down with Subway and just start eating it. If you want to eat Subway, go and sit and eat it at Subway."

He said people saw hospitality businesses as "extensions of their lounge where they can just go hang out. It's a strange mentality."

He had no problem with parents bringing some food for toddlers.

"But we'll have families come in with kids in the 8, 10, 12 age range and the parents order a drink then they basically bust out the kid's lunch box. Usually they're pretty surprised when I say 'I'm sorry, this is not a public place. If you're going to sit here we need you to consume food that you've purchased on the premises'."

He said if someone with a food allergy makes their condition known they try to cook something for that person or in extreme allergy cases where a person can go into anaphylactic shock, his restaurants will heat a sealed meal from home.

However, doing so created an element of risk for the restaurant if the customer got sick.

"In those situations it's not their fault. They want to enjoy time with friends and family so we will look after them as best we can. That's a lot different from going and buying a steak pie at BP and coming and sitting on our leaners."